One of the major limitations of crop productivity is the effect of environmental stress conditions on plant growth and development. An important goal of molecular biology is the identification and isolation of genes that can provide resistance or tolerance to such stresses. For agriculture, the creation of transgenic plants containing such genes provides the potential for improving the stress resistance or tolerance of plants.
Drought, salt loading, and freezing are stresses that cause adverse effects on the growth of plants and the productivity of crops. The physiological response to these stresses arises out of changes in cellular gene expression. Expression of a number of genes has been demonstrated to be induced by these stresses (Zhu et al., 1997; Shinozaki et al., 1996; Thomashow, 1994). The products of these genes can be classified into two groups: those that directly protect against environmental stresses and those that regulate gene expression and signal transduction in the stress response. The first group includes proteins that likely function by protecting cells from dehydration, such as the enzymes required for biosynthesis of various osmoprotectants, late-embryogenesis-abundant (LEA) proteins, antifreeze proteins, chaperones, and detoxification enzymes (Shinozaki et al., 1997, Ingram et al., 1996, Bray et al., 1997). The second group of gene products includes transcription factors, protein kinases, and enzymes involved in phosphoinositide metabolism (Shinozaki et al., 1997). An overview of the methods known to improve stress tolerance in plants is also given in Holmberg & Bülow, (1998).
Further studies are definitely needed to give an insight into the mechanisms involved in the plant response to environmental stress conditions.
The study of plants naturally adapted to extreme desiccation has led to the hypothesis that the genetic information for tolerance to environmental stress conditions exists in all higher plants. In glycophytes, this information would only be expressed in seeds and pollen grains which undergo a desiccation process.
The induction of osmotolerance in plants is very important to crop productivity: 30 to 50% of the land under irrigation is presently affected by salinity. Several lines of evidence also demonstrate that even mild environmental stress conditions throughout the growth season have a negative impact on plant growth and crop productivity. It is for instance known that even minor limitations in water availability cause a reduced photosynthetic rate. Unpredictable rainfall, increase in soil salinity at the beginning and the end of the growing season often result in decreased plant growth and crop productivity. These environmental factors share at least one element of stress and that is water deficit or dehydration. Drought is a significant problem in agriculture today. Over the last 40 years, for example, drought accounted for 74% of the total US crop losses of corn. To sustain productivity under adverse environmental conditions, it is important to provide crops with a genetic basis for coping with water deficit, for example by breeding water retention and tolerance mechanisms into crops so that they can grow and yield under these adverse conditions.